A smart door and a smart dog. Jack lived alone in a big city. He was a careless man. Sometimes he came to the office only to find that the door was unlocked. He had forgotten to lock the door when he left. He also left his keys in the office very often. One day, Jack left his keys in the office. Once again, it would take him nearly an hour to get to his office by subway. He was so tired that he paid a man to unlock the door of his house. In the end, the next day, he replaced the door with a new smart one he could open or lock the door via a mobile app. So he didn't need to worry about forgetting to take the keys with him anymore. However, the other day when he came back home and was about to open the door, he found that his bag had been left in his office, his keys and cell phone were both in the bag. In the end, Jack gave up the smart door. He kept a pet dog and taught it to open the door for him. His pet dog was very clever when he was sure that it was Jack knocking, he always opened